Free: A Knight in Atlantis by Diana Bold

When Sebastian of Hawkesmere returns from the Crusades he becomes a recluse, locking himself away in an enchanted tower, dabbling in alchemy and magic. When his brother captures a beautiful woman with a satchel full of amazing instruments, Sebastian is enlisted to help discern their purpose and discover where she came from. Rhoswen of Atlantis is unlike anyone Sebastian has ever known, and the answers he seeks will shatter his beliefs and force him to choose between love and duty.